Fuseheads

ABSTRACT

A dipped fusehead has its dipped head of ignitible material inserted into a protective cap of heat-shrinkable plastics material, for example irradiated thermally stabilized polyethylene, which is heat-shrunk around and into close contact with the head and is sealed around the body of the fusehead by means of a polyethylene sealing ring in the open mouth of the cap.

United States Patent 1 Ward [ Feb. 20, 1973 |54| FUSEHEADS [75] Inventor: Norman Reginald Ward, Stcvcnagc,

England [73] Assignee: British Aircraft Corporation Limited, London, England [22] Filed: March 16, 1971 [21] Appl. No.: 124,761 [30] Foreign Application Priority Data Mar. 16,1970 Great Britain v.12550/70 52 us. 01. .;...'...I.".' ..1'02/28 M [51] Int. Cl ..F42b 3/12 [58] Field of Search ..l02/28 M [56] References Cited UNITED STATES PATENTS 2,506,229 5/l950 Lowe et al. ..l02/28 M 3,332,350 7/1967 Morfy ..l02/28 M Primary ExaminerVer|in R. Pendegrass Attorney-Cushman, Darby & Cushman [57] ABSTRACT A dipped fusehead has its dipped head of ignitible material inserted into a protective cap of heat-shrinkable plastics material, for example irradiated thermally stabilized polyethylene, which is heat-shrunk around and into close contact with the head and is sealed around the body of the fusehead by means of a polyethylene sealing ring in the open mouth of the cap.

9 Claims, 2 Drawing Figures PATENTED FEB 2 0 I973 INVENTOR amnu REQINQLQ Wmm.

TTORNEYS FUSEHEADS This invention relates to fuseheads for explosive or pyrotechnic charges, and in particular to fuseheads of a type known as dipped fuseheads.

A dipped fusehead for use in conjunction with an explosive or pyrotechnic charge, for which it provides the ignition source, includes a length of high-resistance wire commonly known as a bridgewire, the ends of which are extended to lie on either side of a board of insulating material around an edge of which the wire passes, and are each connected to a respective contact for connection to an electrical source. The wire and the adjacent end of the board are dipped into a substance having low-temperature ignition properties so that a head is formed in a manner similar to that whereby the heads of matches are formed. This is called the primary dip. The head is then dipped into a second ignitable substance which forms a hard casing round the first material. This is called the secondary dip. After the primary and secondary dips the dipped head may be given a varnish dip.

Such dipped fuseheads are typically provided in explosive cartridges. It has been found however that there is a tendency for certain of the plasticizers used in the manufacture of explosive cartridges to migrate into the dipped fuseheads of the cartridges, thereby causing softening and swelling of the fuseheads and a risk of subsequent failure to ignite the cartridges. Dampness in the atmosphere may also cause softening and swelling of the fuseheads. In addition, if a fusehead casing is damaged the casing may, upon firing, split open thereby preventing the build-up of temperature and pressure within the casing which is necessary for correct ignition or detonation of the explosive charge in the cartridge.

An object of the present invention is to provide a dipped fusehead in which these difficulties and disadvantages are avoided by the enclosure of at least the ignitable head of the fusehead in a protective cap of plastics material.

According to the invention, a dipped fusehead is provided with a protective cap of heat-shrinkable plastics material, for example irradiated thermally stabilized polyolefin, which cap is shrunk about at least the dipped head of ignitable material of the fusehead in close contact therewith.

In a preferred form of the invention, a sealing ring of low-melting-point material such as polythene is provided surrounding the fusehead within the cap and adjacent to its open end, the sealing ring forming a substantially air-tight seal around the fusehead and between it and the cap.

The invention from another aspect comprises a method of capping a dipped fusehead in which method at least the dipped head of the fusehead is inserted into the interior of a protective cap made of a heat-shrinkable plastics material, for example irradiated thermally stabilized polyolefin, and heat is applied to the cap to shrink it into close contact with the fusehead.

The invention may be carried into practice in various ways, but one specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 shows a dipped fusehead provided with a protective cap, part of the drawing being shown in section, and

FIG. 2 shows a typical protective cap before applica' tion to a dipped fusehead.

In the illustrated embodiment a length of electrical resistance wire 1 is positioned about a base board 3 of insulating material so that one end of the wire makes contact with an electrical contact pad 4 on one side of and near one end of the board and the other end of the wire makes contact with a similar electrical contact pad 5 similarly situated on the reverse side of the board, the wire passing around the edge of the board at the other end of the board. Electrical leads 6, 7 are provided for connecting the two pads 4, 5, to a source of electrical power. A head 8 of ignitable material is formed by double dipping in a known manner about the said other end part of the board 3 and the central part of .thewire passing around its edge.

Enclosing the dipped head 8 is a protective cap 9 which fits tightly around the head 8 and forms a tight fit around the board 3 and associated electrical pads 4 and 5. The protective cap 9 is shown separately in FIG. 2, and is of irradiated heat-shrinkable thermally stabilized polyolefin sheet 0.010 inch thick, having a length of 0.30 inch and an internal diameter of 0.125 inch. A sealing ring 11 made of a low-melting-point plastics material, e.g., polyethylene (not thermally stabilized), is located in an annular recess 12 in the neck of the cap.

A suitable material for the heat-shrinkable cap 9 is irradiated polyethylene, although polypropylene, polytetrafluoroethylene or polychlorotrifluoroethylene, in each case thermally stabilized by irradiation, may also be used.

During manufacture of a fusehead, and after the ig' nitable head 8 has been formed by double dipping, the head of the fusehead is inserted into a cap 9 which is then shrunk into close contact with the head by being subjected to heat at a temperature of C for 3 minutes. The usual varnish clip of the head of the fuse, which follows the primary and secondary dips in the manufacture of conventional fuseheads, may be omitted if desired in the present case. The resultant product is a fusehead whose head 8 is covered by a close fitting cap of plastics material which is sealed around the base board 3 by a close-fitting seal formed by the sealing ring 11, which is softened and pressed into contact with the base board beyond the head 8 by the heat-shrinkage of the cap 9.

What we claim as our invention and desire to secure by Letters Patent is:

l. A dipped electrical fusehead which comprises a high-resistant bridgewire mounted to a support and embedded in an ignition head, said ignition head compris ing a primary dip coating of ignitable material having low-temperature ignition properties and a secondary dip coating of a hardenable ignitable material, said secondary dip coating forming a hard casing around said primary dip coating, and said ignition head being inserted in and wholly enclosed in a sealed protective cap made of heat-shrunk plastics material, said cap having one open end and one closed end and being heat-shrunk around the entire ignition head in closefitting contact therewith over the whole external surface of said head, the support protruding through said open end of the protective cap which is shrunk into sealing engagement around and with said support.

2. A dipped fusehead as claimed in claim 1 in which the plastics material forming the protective cap is an irradiated thermally stabilized polyolefin.

3. A dipped fusehead as claimed in claim 2 in which the polyolefin is polyethylene.

4. A dipped electrical fusehead which comprises a high-resistance bridgewire mounted to a support and embedded in an ignition head, said ignition head comprising a primary dip coating of ignitable material having low-temperature ignition properties and a secondary dip coating of a hardenable ignitable substance, said secondary dip coating forming a hard casing around said primary dip coating, and said ignition head being inserted in and wholly enclosed by a sealed protective cap made of heat-shrunk plastics material, said cap having one open end and one closed end and being heat-shrunk around the entire ignition head in close fitting contact therewith, and a sealing ring of low-melting-point material provided surrounding the support within the cap and adjacent to its open end and forming a substantially air-tight seal around the support and between it and the heat-shrunk cap.

5. A dipped fusehead as claimed in claim 4 wherein the sealing ring is of polyethylene.

6. A method of making a dipped fusehead which comprises mounting a high-resistance bridgewire to a support, embedding the bridgewire in an ignition head by first forming a primary dip coating of ignitable material having low-temperature ignition properties around the bridgewire and then forming a secondary dip coating of a hardenable ignitable substance around said first dip coating, permitting said secondary dip coating to harden thereby forming a hard casing around said primary dip coating, inserting the ignition head into the interior of a protective cap made of a heat-shrinkable plastics material, said protective cap having an open end and a closed end and having a sealing ring of low-melting-point material provided within the cap and adjacent to its open end, through which sealing ring the ignition head is inserted, and then applying heat to the protective cap to cause it to shrink into close contact with the ignition head, the said sealing ring becoming softened by the said application of heat and forming a substantially air-tight seal around the support and between it and the shrunk cap.

7. A method as claimed in claim 6 in which the plastics material of the protective cap is an irradiated thermally stabilized polyolefin. I

8. A method as claimed in claim 7 in which the polyolefin is polyethylene.

9. A method as claimed in claim 6 wherein the sealing ring is made of polyethylene. 

1. A dipped electrical fusehead which comprises a high-resistant bridgewire mounted to a support and embedded in an ignition head, said ignition head comprising a primary dip coating of ignitable material having low-temperature ignition properties and a secondary dip coating of a hardenable ignitable material, said secondary dip coating forming a hard casing around said primary dip coating, and said ignition head being inserted in and wholly enclosed in a sealed protective cap made of heat-shrunk plastics material, said cap having one open end and one closed end and being heat-shrunk around the entire ignition head in close-fitting contact therewith over the whole external surface of said head, the support protruding through said open end of the protective cap which is shrunk into sealing engagement around and with said support.
 2. A dipped fusehead as claimed in claim 1 in which the plastics material forming the protective cap is an irradiated thermally stabilized polyolefin.
 3. A dipped fusehead as claimed in claim 2 in which the polyolefin is polyethylene.
 4. A dipped electrical fusehead which comprises a high-resistance bridgewire mounted to a support and embedded in an ignition head, said ignition head comprising a primary dip coating of ignitable material having low-temperature ignition properties and a secondary dip coating of a hardenable ignitable substance, said secondary dip coating forming a hard casing aRound said primary dip coating, and said ignition head being inserted in and wholly enclosed by a sealed protective cap made of heat-shrunk plastics material, said cap having one open end and one closed end and being heat-shrunk around the entire ignition head in close fitting contact therewith, and a sealing ring of low-melting-point material provided surrounding the support within the cap and adjacent to its open end and forming a substantially air-tight seal around the support and between it and the heat-shrunk cap.
 5. A dipped fusehead as claimed in claim 4 wherein the sealing ring is of polyethylene.
 6. A method of making a dipped fusehead which comprises mounting a high-resistance bridgewire to a support, embedding the bridgewire in an ignition head by first forming a primary dip coating of ignitable material having low-temperature ignition properties around the bridgewire and then forming a secondary dip coating of a hardenable ignitable substance around said first dip coating, permitting said secondary dip coating to harden thereby forming a hard casing around said primary dip coating, inserting the ignition head into the interior of a protective cap made of a heat-shrinkable plastics material, said protective cap having an open end and a closed end and having a sealing ring of low-melting-point material provided within the cap and adjacent to its open end, through which sealing ring the ignition head is inserted, and then applying heat to the protective cap to cause it to shrink into close contact with the ignition head, the said sealing ring becoming softened by the said application of heat and forming a substantially air-tight seal around the support and between it and the shrunk cap.
 7. A method as claimed in claim 6 in which the plastics material of the protective cap is an irradiated thermally stabilized polyolefin.
 8. A method as claimed in claim 7 in which the polyolefin is polyethylene. 